During the last two decades, it has been shown that cells can directly communicate with each other. That is, there is direct transfer of electrical signals and small molecules from one cell cytoplasm to adjacent cell cytoplasms. Various details governing this type of cell communication have been elucidated. Only in the last few years have the cells of the mammalian inner ear been included in this research effort. In fact, the existence of cell communication in the inner ear was only recently proved (Santos-Sacchi and Dallos, 1982, 1983); and some of the factors governing coupling in the organ of Corti have now been studied in vitro (Santos-Sacchi, 1984a,b, 1985a,b, 1986a,b). In addition, differences have been found between the ability of supporting cells to communicate in the in vitro and in vivo preparations, such that it is thought (Santos-Sacchi, 1986e) that coupling ratios and space constants are kept small in vivo by the inner ear's normal physiologic compartmentalization. The aim of this project is to determine what factors (e.g., membrane potential, pH, auditory stimulation, various ions) are important for cell communication in vivo and in vitro, and to determine what aspect of the normal compartmentalization of the inner ear maintains poor electrical coupling (e.g., endolymphatic potential, unique ionic makeup of the cochlear fluids). Furthermore, dye coupling studies will be performed in vivo, since it is now known that dye coupling occurs in vitro (Santos-Sacchi, 1986c,d). Finally, the ultrastructure of supporting cell gap junctions will be studied to determine if there are structural correlates of physiological coupling differences found in vitro versus in vivo. The microenvironment is critical for the normal functioning of the organ of Corti. The results of this study may provide information as to the role of cell-to-cell coupling in the normal function of the organ of Corti, and may also be significant in understanding pathologic effects upon the organ under varying disturbances in pH and ionic conditions. In addition, it may provide insight into the effects of noise exposure upon the inner ear.